front axle leak

So, the CAD side of the axle is leaking differential oil ?
Running down the brake disc and inside of the tire/wheel..?
Just how steep was that side-hill maneuver ?

Only when you put it in 4wd? That's kind of odd. Just out of curiosity, did you put manual hubs on? The reason I ask, is if you haven't, your axles are spinning all of the time anyway so I can't see why 4wd would change anything. Still kind weird.
Regardless, it sounds like your axle seal needs to be replaced. You need to remove your: Brake caliper, hub/rotor, caliper adapter, spindle then the front shaft. Then just remove the seal on the end of the axle tube, it's quite easy to do.
Obviously, while you're there, replace all seals involved and your u-joint if it ain't new.
One more thing, the parts stores often have a hard time finding that seal #, so go to our handy dandy part number index to find it.

actually, the plot thickens! My rear drive shaft fell out when I busted a u-joint. so I shifted into 4x and got off the street. After towing it home, I saw the gear oil all over the inside of my tire. At first I was thinking my brake line. But then as I bent over to investigate I got that unmistakable yummy smell of gear oil.

Sounds easy, though.
Thanks!

It isn't that hard, it will just take you a while, might as well take a look at the ball joints while you have it apart.